The present invention generally relates to charged particle beam exposure methods and apparatuses, and more particularly to a charged particle beam exposure method and a a charged particle beam exposure apparatus which use a plurality of charged particle beam columns.
Recently, due to the high integration densities of semiconductor integrated circuits, the exposure techniques (lithography techniques) used to form patterns on a semiconductor wafer are changing from the photolithography techniques which were primarily used in the past to the charged particle beam exposure techniques which employ a charged particle beam typified by the electron beam. The charged particle beam exposure techniques are capable of exposing extremely fine patterns.
The charged particle beam exposure techniques include the so-called variable rectangular exposure technique, the block exposure technique and the like depending on the shapes of the patterns that can be generated at one time. According to the variable rectangular exposure technique, the size of the rectangles that are exposed is varied, and a desired pattern is exposed by successively exposing the variable size rectangles. On the other hand, according to the block exposure technique, the charged particle beam is transmitted through a mask having repeating basic unit patterns, and even a complicated pattern is exposed in one shot using a unit pattern. Hence, the block exposure technique is particularly effective when the pattern is extremely fine but virtually the entire area to be exposed is a repetition of the basic pattern, such as the case of a 256 Mbit dynamic random access memory (DRAM) pattern.
FIG. 1 is a diagram showing an example of a conventional electron beam exposure apparatus employing the block exposure. The electron beam exposure apparatus includes an electron gun 101, an electron lens system L1a, a plate 102 with a rectangular aperture, an electron lens system L1b, a beam shaping deflector 103, a first mask deflector MD1, a dynamic mask stigmator DS, a second mask deflector MD2, a dynamic mask focus coil DF, an electron lens system L2a, a mask stage 105 on which a block mask 104 is mounted, an electron lens system L2b, a third mask deflector MD3, a blanking deflector 106, a fourth mask deflector MD4, a reduction electron lens system L3, a circular aperture stop 107, a projection electron lens system L4, a main deflector (electromagnetic deflector) 108, a sub deflector (electrostatic deflector) 109, a projection electron lens system L5, a wafer stage 111 on which a wafer 110 is mounted, and a control system 131. Those parts other than the control system 131 form an electron beam column 130 of the electron beam exposure apparatus.
On the other hand, the control system 131 includes a central processing unit (CPU) 121, a clock unit 122 which generates various clock signals including an exposure clock, a buffer memory 123, a control unit 124, a data correction unit 125, a mask memory 126, and a main deflector setting unit 127. The CPU 121 which controls the operation of the entire electron beam exposure apparatus, the clock unit 122, the mask memory 126 and the main deflector setting unit 127 are coupled via a bus 128. In FIG. 1, it is assumed for the sake of convenience that the data correction unit 125 and the main deflector setting unit 127 include the functions of a digital-to-analog converter (DAC) and an analog amplifier. A laser interferometer which measures the position of the wafer stage 111, and a stage moving mechanism which moves the wafer stage 111 are respectively known from the disclosures of U.S. Pat. No. 5,173,582 and U.S. Pat. No. 5,194,741, for example, and an illustration and description thereof will be omitted in this specification.
An electron beam emitted from the electron gun 101 passes through the plate 102 and is deflected by the first and second deflectors MD1 and MD2 so as to pass a desired pattern portion on the block mask 104. The cross sectional shape of the electron beam is shaped by the pattern portion, and this electron beam is returned to the optical axis by the converging functions of the electron lens systems L2a and L2b and the deflecting functions of the third and fourth deflectors MD3 and MD4. Thereafter, the cross sectional area of the electron beam is reduced by the reduction electron lens system L3, and is irradiated on the wafer 110 via the projection electron lens systems L4 and L5, so as to expose the desired pattern on the wafer 110.
The buffer memory 123 stores exposure pattern data related to the patterns to be exposed on the wafer 110, block data related to mask patterns on the block mask 104 and the like. The exposure pattern data, the block data and the like are stored into the buffer memory 123 from a memory 129 of the CPU 121, which memory 129 is coupled to the bus 128. The exposure pattern data includes main deflection data to be supplied to the main deflector 108 and the like. In addition, the mask memory 126 stores data related to the relationships of the mask pattern positions that are measured in advance prior to the exposure and the deflection data, correction data for correcting the deflection data to be supplied to the dynamic mask stigmator DS and the dynamic mask focus coil DF and the like.
The exposure pattern data which are input by the CPU 121 and stored in the buffer memory 123 include pattern data codes PDC which indicate the mask patterns on the block mask 104 to be used for the exposure. The control unit 124 uses the pattern data code PDC, reads the deflection data for deflecting the electron beam to the position of the mask pattern to be used, and supplies the read deflection data to the first through fourth deflectors MD1 through MD4. In addition, the deflection data read from the mask memory 126 are also supplied to the data correction unit 125. The deflection data are read from the mask memory 126 based on the exposure clock which is generated from the clock unit 122.
On the other hand, the main deflector setting unit 127 reads the main deflection data from the buffer memory 123 based on the clock received from the clock unit 122, and supplies the read main deflection data to the main deflector 108. In addition, the deflection data of the sub deflector 109, the deflection data of the beam shaping deflector 103 and the deflection data of the blanking deflector 106 are decomposed into shot data in the control unit 124 depending on the data stored in the buffer memory 123. The shot data is supplied to the corresponding sub deflector 109, beam shaping deflector 103 and blanking deflector 106 via the data correction unit 125. In other words, the control unit 124 obtains the size of the electron beam when making the variable rectangular exposure and the deflection position of the electron beam on the block mask 104 depending on the data stored in the buffer memory 123, and supplies the size and deflection position information to the data correction unit 125. Each deflection data of the electron beam dependent on the pattern to be exposed and supplied from the control unit 124 is corrected in the data correction unit 125 based on the correction data read from the mask memory 126. The deflection data of the beam shaping deflector 103 determine the variable rectangular size of the cross section of the electron beam, and the deflection data of the blanking deflector 106 are set for each exposure shot.
FIGS. 2A and 2B are diagrams for explaining an example of the block mask 104 that is used to expose patterns of a memory. As shown in FIG. 2A, the block mask 104 is made up of a substrate 104a which is made of a semiconductor such as silicon or a metal, and a plurality of deflection areas 104-1 through 104-12 which are provided on this substrate 104a. A plurality of mask patterns are formed in each of the deflection areas 104-1 through 104-12. In the electron beam exposure apparatus which employs the block exposure, a range of the mask patterns that are selectable by deflecting the electron beam about a position on a certain mask stage 105 is fixed, and each of the deflection areas 104-1 through 104-12 have a range of a square having a side of 5 mm, for example, in correspondence with this range of the selectable patterns. For example, when carrying out the exposure by selecting a mask pattern within the deflection area 104-8, the mask stage 105 is moved so that the electron optical axis of the electron beam exposure apparatus approximately matches the center of the deflection area 104-8.
FIG. 2B shows the construction of the deflection area 104-8. For example, 48 block patterns can be arranged within this deflection area 104-8, and each block pattern can be recognized by the pattern data code PDC. In other words, the pattern data code PDC is an index for reading the contents of the mask memory 126 corresponding to each mask pattern based on the exposure clock from the clock unit 122, which exposure clock has a maximum frequency of 10 MHz, for example. As described above, the mask memory 126 stores data related to the relationships of the mask pattern positions that are measured in advance prior to the exposure and the deflection data, correction data for correcting the deflection data to be supplied to the dynamic mask stigmator DS and the dynamic mask focus coil DF and the like, for the purposes of deflecting the electron beam to each mask pattern position. These data are stored in the mask memory 126 by adjusting the electron beam in advance prior to the exposure, and obtaining the deflection data, correction data and the like with respect to the deflection area that is to be used.
As described above, in the electron beam exposure, the desired pattern is exposed on the wafer 110 by deflecting the electron beam to scan depending on the exposure pattern data. In this case, the number of times the electron beam is irradiated is on the order of 10 Mshots/chip or 1 Gshot/chip, and the period of the electron beam irradiation is approximately 100 ns. Such an extremely large number of shots must be accurately irradiated at a high speed to a predetermined position with a predetermined intensity. In addition, this exposure operation must continue in a stable manner.
However, the electron beam exposure apparatus is made up of mechanical parts such as the electron optical column and stage, and hardware parts such as the digital controller and analog amplifier. For this reason, it is impossible to avoid generation of an abnormality in the electron beam exposure apparatus. The abnormalities generated in the electron beam exposure apparatus include the beam fluctuation and the shot dropout.
The beam fluctuation in most cases is primarily caused by the discharge at the high voltage power supply and the electron gun part, the noise at the lens power supply, the effects of the external electromagnetic noise, the charge-up of the electron beam column, and the like. In other words, the cause of the beam fluctuation in most cases is found on the side of the electron beam column. In addition, the fail time varies from several .mu.s to several hundred ms depending on the cause of the beam fluctuation, and the repetition frequency is various.
On the other hand, the shot dropout in most cases is primarily generated by the bit dropout, latch error and the like at the digital circuit and the digital circuit part such as the DAC. The shot dropout varies from a positional error of 1 shot to a positional error in units of patterns.
In the conventional electron beam exposure apparatus, the generation of the above described abnormalities in the electron beam exposure apparatus is only detected after inspecting the exposed patterns. In addition, when analyzing the cause of the abnormality, there was a problem in that the abnormality must be confirmed while monitoring the operation of the part which is considered to be the cause and waiting for the abnormality to actually occur.
In addition, in order to accurately determine the cause of the abnormality in the electron beam exposure apparatus, it was normally necessary to generate the abnormality several tens of times. However, in the case of the abnormality which rarely occurs, it took an extremely long time to analyze the abnormality, and there was a problem in that the operating efficiency of the electron beam exposure apparatus greatly deteriorates.